Internal combustion engines such as gasoline engines, diesel engines, and gaseous fuel-powered engines exhaust a complex mixture of air pollutants. These air pollutants are composed of solid particulate matter and gaseous compounds including nitrous oxides (NOx). Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of solid particulate matter and gaseous compounds emitted to the atmosphere from an engine is regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions has been to implement exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas, which is redirected to a cylinder of the engine, reduces the concentration of oxygen therein, thereby increasing the heat capacity of the mixture and lowering the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature and reduced oxygen slow the chemical reactions responsible for the formation of NOx, thereby reducing the amount of NOx emitted by the engine. In addition, the particulate matter entrained in the exhaust is burned upon reintroduction into the engine cylinder to further reduce the exhaust gas by-products.
One available type of EGR system is called a low pressure system. Low pressure EGR systems draw low pressure exhaust from downstream of an engine's turbine and direct the exhaust to a location upstream of the engine's compressor. An example of a low pressure EGR system was disclosed in U.S. Pat. Publication No. 2006/0156724 (the '724 publication) by Dismon et al. on Jul. 20, 2006. Specifically, the '724 publication disclosed an exhaust gas return system having a particulate trap located in series with and downstream of a turbine. The exhaust gas return system also has a catalyst located in series with and downstream of the particulate trap. Exhaust gas is drawn from a location between the particulate filter and the catalyst for return to an air inlet passageway upstream of a compressor. An exhaust gas return valve is disposed within an exhaust gas line between the particulate filter and the catalyst to control the flow rate of returned exhaust gases.
Although the low pressure exhaust gas return system of the '724 publication may reduce the amount of NOx and particulate matter exhausted to the atmosphere, it may be limited. In particular, there may be some situations where the pressure differential between the exhaust and intake air is insufficient for proper operation. In other words, it is possible for the pressure of the recirculated exhaust to be substantially the same as or even lower than the pressure of the intake air. In these situations, the exhaust will flow poorly or not at all into the air inlet passageway. Without sufficient return of the exhaust, the engine's emissions may fail to be compliant with the environmental regulations.
Further, the disclosed placement of the exhaust gas return valve may be problematic. Specifically, because this valve is located within the exhaust gas line, the temperatures experienced by the valve may be excessive. These high temperatures may degrade the valve over time, possibly resulting in premature failure of the valve.
The disclosed EGR system is directed to overcoming one or more of the problems set forth above.